1. Field of the Invention
The present invention concerns a method for acquisition of magnetic resonance image data, as well as a magnetic resonance apparatus operable according to such a method.
2. Description of the Prior Art
Magnetic resonance (MR) imaging is a known, firmly established modality that is particularly used in medical imaging. A body to be examined is introduced into a strong, homogeneous static magnetic field (known as the basic magnetic field) that causes an alignment of the nuclear spins of atomic nuclei in the body, in particular of hydrogen atomic nuclei (protons) bound to water. These nuclei are excited to a precessional movement around direction of the basic magnetic field by means of radio-frequency excitation pulses. After the end of a radio-frequency (RF) excitation pulse, the nuclear spins precess at a frequency that is known as the Larmor frequency, which depends on the strength of the basic magnetic field. Due to various interaction types, the nuclear spins align (with a characteristic time curve) again along the preferred direction provided by the basic magnetic field. The time curve is, among other things, tissue-dependent and can be described using a characteristic known as the relaxation time. By computational and/or measurement analysis of the integral, radio-frequency magnetic resonance signals, an image can be generated from the spatial distribution of the spin density in connection with the respective relaxation times. The association of the nuclear magnetic resonance signals (that can be detected as a consequence of the precessional movement) with the location or their origin ensues by the application of magnetic field gradients. For this purpose, gradient fields are superimposed on the basic magnetic field and controlled such that an excitation of the nuclei ensues only in a slice to be imaged. An RF coil device is required both for RF excitation of the nuclear spins and for detection of the nuclear response signals. Imaging systems based on these physical effects are known as magnetic resonance imaging (MRI) systems.
In MR, the acquired measurement signals and the image data to be reconstructed therefrom are linked via a Fourier transformation. For example, in order to acquire tomographic image data of a slice of a subject to be examined, a two-dimensional space (known as k-space) is sampled by a series of measurement signals and the image is subsequently reconstructed by a Fourier transformation of a two-dimensional k-space matrix determined from the measurement signals Since a measurement signal typically arises from the radiation of an excitation pulse, the activation of diverse gradient fields, and the acquisition of the decaying transverse magnetization of the nuclear spins, it can sometimes take a relatively long time until the two-dimensional k-space matrix has been sampled at a resolution necessary for the image quality.
In order to address the problem of long acquisition time, various methods that are known as “single shot” methods have been developed. In these methods, k-space is scanned with the measurement signal after a single excitation pulse (“single shot”) by skillful switching of the gradient fields, RF pulses or a combination of these, such that the image data can be acquired in a shorter time.
These methods are known, among other things, as gradient echo sequences or EPI (echoplanar imaging) sequences, spin echo sequences and GRASE (gradient and spin echo) sequences. A disadvantage of the cited methods is the often insufficient resolution with which k-space is covered, such that the image data do not always exhibit the necessary degree of detail. The “single shot” methods, in particular EPI sequences, are additionally prone to susceptibility and eddy current artifacts that in part significantly reduce the quality of the reconstructed images.
Methods known as “multi-shot” methods represent a compromise between fast acquisition and good image quality. In such methods, k-space is scanned not with a single excitation pulse (with successive acquisition of the measurement signal) but rather successively in segments using a number of excitation pulses. By such segmented scanning, the entirety of k-space is canned by specific segments of the k-space matrix being respectively scanned upon each excitation pulse. The individual segments can be scanned in this manner with a greater precision than given the “single shot” methods. The image can then be reconstructed, for example, from the measurement signals by Fourier transforming the individual k-space segments into partial images, and the partial images are subsequently added.
A problem that occurs in “multi-shot” methods is an increased sensitivity to movement artifacts. In particular, the phase of the transverse magnetization of the excited spins changes when excited spins move along magnetic field gradients. Given a simple movement, such as a translation, during the measurement signal acquisition after an excitation pulse, a global phase shift that is reflected as a global phase shift in image space is impressed on the transverse magnetization. In the case of more complex movements during the measurement signal acquisition (for example pulsing expansions caused by the blood flow), a locally-varying phase distribution that is reflected in the reconstruction image as a locally-varying phase distribution is impressed on the transverse magnetization.
In “single shot” methods, a locally varying phase distribution in the reconstructed image typically does not represent a problem, since primarily the magnitude of the transverse magnetization of the spins is decisive for the reconstructed image. However, when “multi-shot” methods are used, the entire image is calculated from the individual partial images corresponding to the k-space segments. If the various partial images respectively exhibit different phase distributions since the subject to be examined had different movement patterns at the respective acquisition points in time of the associated k-space segments, the different phase distributions lead to significant disruptive interferences in the image reconstruction.
Strong field gradients (primarily given high diffusion values) are activated in diffusion-weighted imaging, in which “multi-shot” methods are typically applied. A particularly distinct phase distribution is thereby impressed on the transverse magnetization of a moving subject, such that the artifacts just described occur particularly prominently due to the movement-induced different phase distribution in the individual partial images.
One possibility to compensate for this type of movement-induced artifacts is the use of what are known as navigator signals or navigator echoes. A navigator signal means a measurement signal that is acquired after each excitation pulse in addition to the measurement signal of the k-space segment, and with which the same (advantageously central) segment of k-space is always scanned.
The measurement signals of the different k-space segments cannot be directly compared with one another since the k-space segments typically do not overlap. The navigator signals that always acquire the same k-space segment, however, can be compared with one another due to this. The phase distribution that has occurred upon the respective acquisition of the associated k-space segments therefore can be determined by the evaluation of the navigator signals and through their comparison. The partial images belonging to the k-space segments thus can be phase-corrected and can be added largely without interference in the reconstruction of the image.
Although this correction method delivers considerably better images than a reconstruction without consideration of the phase distribution, this method has its limitations. The navigator signal typically covers only a relatively small central region of the k-space matrix. The phase distribution calculated from this region therefore reflects only the low-frequency components. If the movement of the subject during the acquisition of a measurement signal and navigator signal was complex, such that the partial image exhibits a strongly-varying phase distribution (thus a phase distribution with impressed radio-frequency components), it can occur that this phase distribution is determined only insufficiently by the navigator signal.
In “The use of intelligent re-acquisition to reduce scan time in MRI degraded by motion” in Proceedings, ISMRM, 6th Annual Meeting, Sydney, Australia 1998, p. 134, Nguyen Q, Clemence M, Ordidge R J disclose a method with which the diffusion-weighted images can be improved using navigator echoes with regard to distortions due to ghost images. The navigator echo disclosed in this article is a one-dimensional navigator echo in the frequency coding direction. According to this article, the integrated navigator echo supplies a measure that indicates the image-to-ghost image ratio, and thus characterizes the quality of the image. The method furthermore implements diverse acquisition algorithms with which those echo signals whose navigator signals suggest a particularly disadvantageous image-to-ghost image ratio are repeatedly acquired.
A method based on this technique is disclosed by Nguyen Q, Thornton J, Ordidge R J in “sotropic diffusion-weighted multishot imaging using automatic reacquisition” in Proceedings, ISMRM, 7th Annual Meeting, Philadelphia, USA 1998, p. 559. A diffusion-weighted image is thereby acquired with a “multi-shot” spin echo sequence with navigator echoes. After acquisition of the measurement signals that navigator echo that characterizes a particularly low ghost image ratio is identified from the various navigator echoes. Using this navigator echo those echoes are now identified that cause the largest ghost image artifacts in the image. These echoes are thereupon reacquired.
The two articles do in fact disclose a method with which specifically those echoes can be identified that lead to ghost artifacts, such that these echoes can be reacquired. Other types of artifacts that, for example, are caused by a radio-frequency phase distribution in the individual partial images are still not accounted for by this method, such that improving the image quality is still desirable.